Add logging to AltchaField for validation failures and misconfigurations

[hleroy]

Adds a module-level logger and a log_warning() method on AltchaField (overridable by subclasses) that emits warnings in the following cases:

• ALTCHA_VERIFICATION_ENABLED is False — silent bypass of CAPTCHA protection is now visible in logs
• altcha.verify_solution raises an unexpected exception
• Token verification fails (with the reason returned by the library)
• Payload base64/JSON decoding fails
• A replay attack is detected

Without logging, all these failure paths were silent, making it difficult to distinguish misconfiguration from attacks or bugs in production.